Blender Mesh to UE4: Correct normals but missing faces
You might have Normals facing in the right direction, but still get some missing faces in UE. Here's some solutions: 


Try to Triangulate your Mesh in Blender (Add Modifier >> General >> Triangulate) and then Export it. 
Apply all Transformations in Blender before exporting your model (in Object Mode, go to the Object >> Apply >> All Transforms)


By the way, you can also disable Backface Culling in Blender to be able to see your model without displaying Normals direction or color. 

You can also activate double-sided Materials in Unreal Engine, but it will slightly reduce performance (depending upon the mesh itself). 

Hope it helps!